Implantable electrodes are used for cardioverting a human heart by applying electric shocks in vivo to the heart. A variety of arrhythmias of the heart can be treated internally with the use of implantable electrodes. Such arrhythmias can include, for example, atrial fibrillation, atrial flutter, atrial tachycardia, ventricular fibrillation, ventricular tachycardia, and asystole (a stopped heart). The term "defibrillation" or "defibrillating" as used herein, includes the term "cardioversion" and all other terms that denote a method of applying electric shocks to the human heart to treat such arrhythmias.
Typically, implantable electrodes are connected by leads to an implantable standby defibrillator. A typical defibrillator includes a capacitor. The capacitor is charged with and stores an electric charge capable of depolarizing a human heart when discharged across the implantable electrodes. The capacitor is connected in a discharge circuit, which is in turn connected to a sensing circuit. The system may also include a sensor for monitoring one or more physiological activities of the patient. The sensor is typically located in a transvenous pacing lead. When an arrhythmia of the heart is sensed, the capacitor is caused by the discharge circuit to discharge an electric shock which is delivered to the heart through two implantable electrodes.
Various shocking electrode configurations are known. One implantable shocking electrode may be positioned within the right ventricle or right atrium of the heart, with the distal tip of the transvenous lead being used for pacing. The other implantable shocking electrode may be positioned either directly on the ventricular myocardium or subcutaneously positioned along the interior chest wall.
Other known implantable shocking electrode configurations used in conjunction with implantable defibrillators employ electrodes which are all in contact with the exterior surface of the human heart.
The voltage levels and pulse sequences which will successfully terminate a particular tachycardia or fibrillation are not universal, and frequently a number of different combinations must be tried before the arrhythmia is successfully terminated. The voltage levels and pulse sequences may include various morphologies to include biphasic and/or monophasic waveforms. In all of the known implantable electrode systems, an electric current is discharged in a path across a first implantable electrode through the human heart to a second implantable electrode. Therefore, such polarity reversals must be undertaken in an electronic multiplexer containing in the implantable housing, or additional electrodes must be provided. The first approach occupies additional space and consumes additional energy, both of which are undesirable, and the second approach requires more extensive surgery than would otherwise be necessary.